Portable electronic devices are becoming smaller, lighter and sometimes more energy demanding. This has led to an increase of interest in high-capacity and compact batteries. Non-aqueous electrolyte lithium-ion batteries are regarded as one of the most promising technologies for these applications. During lithiation, a lithium is added to the active material, during delithiation a lithium ion is removed from the active material. Most of the currently applied anodes in the lithium ion batteries function by a lithium intercalation and de-intercalation mechanism during charging and discharging. Examples of such materials are graphite and lithium titanium oxide (LTO). However these active anode materials lack high gravimetric and volumetric capacity. The gravimetric capacity of graphite and LTO is 372 mAh/g (LiC6) and 175 mAh/g (Li4Ti5O12) respectively.
Another class of active materials functions by alloying and de-alloying lithium with a metal, metal alloy or a composite metal alloy. The term metal can refer to both metals and metalloids. Several good examples are pure silicon, pure tin or amorphous CoSn alloy that is commercialized by Sony as Nexelion. Problems with the application of lithium alloying type of electrodes is mainly related to the continuous expansion and decrease in volume of the particles or by unwanted phase changes during cycling. Repeated expansion and contraction of the particle volume can create contact loss between the particles and current collector, a decomposition of the electrolyte due to a repeated exposure to a fresh particle surface as the volume changes, a pulverization or cracking of the particle due to internal stress. Phase changes during long term cycling also have an influence. After lithiating pure silicon to the Li15Si4phase the cycling is no longer reversible. Also a presence or creation of a crystalline free tin phase instead of a tin-transition metal alloy phase after delithiation during long term cycling deteriorates the capacity.
The object of this present invention is to provide a negative electrode material for non-aqueous electrolyte secondary batteries with a high capacity and long cycling life.